In the lighting field and, in particular, in the field of light projectors (lighting apparatuses that, by means of optical elements such as mirrors or lenses, project an intense light beam with a great capacity, which is emitted by a light source, so as to light objects and/or rooms), parabolic reflectors are largely used, namely reflectors having a parabolic cross section and obtained from a parabolic profile which, by rotating around its axis, generates a paraboloid of revolution.
By placing a light source in the focus of the rotation solid obtained by so doing (parabolic reflector), a light projector with an extremely narrow and basically collimated beam is obtained.
To change the amplitude of the light beam emitted, thus obtaining light beams with larger emission angles, which is often requested by users, it is known to provide the inner surface of the parabolic reflector with curved (convex) facets arranged in a radial manner. Depending on the bending radius of the facets, beams with different amplitudes are obtained.
Projectors of the type described above are affected by at least two main drawbacks:                the beam emitted by the projector is influenced by the parabolic profile and usually has an excessive axial intensity;        the facets arranged in a radial manner, in order to distribute the light far from the axis and, hence, widen the light beam emitted, must have a very marked curvature (short bending radius), but, in this way, the light reflected by the facets can be redirected into the reflector (instead of being projected outwards), thus causing secondary reflection that can cause dazzling and reduce the optical efficiency.        
Furthermore, if the projector uses a LED light source, which is becoming more and more common, you can typically experience the projection of undesired light rings onto a working surface lit by the projector.
White light LEDs, then, emit light beams that are usually subjected to a colour temperature variation as the distance from the symmetry axis of the LED increases. As a consequence, the further drawback can arise consisting in the projection of yellowish areas (spots) on a white background.
Finally, it should be taken into account the fact that commonly used reflective surfaces normally are mirror-polished and have irregularities due to the production process and, in particular, to the techniques used to obtain the polished finish. These irregularities, which usually do not jeopardize optical performances, can though create an undesired effect, making the reflector “bright” (lit up).